RFID system including a reader/writer and RFID tag

ABSTRACT

An RFID system includes an antenna of a reader/writer and an antenna of an RFID tag. Transmission and reception of a high-frequency signal of a UHF band is performed between the antenna of the reader/writer and the antenna of the RFID tag that are arranged so as to be adjacent to each other. A loop antenna including a loop conductor is used as the antenna of the reader/writer, and coil antennas including a plurality of laminated coil conductors are used as the antenna of an RFID tag. In addition, the conductor width of the loop conductor in the loop antenna is greater than the conductor widths of the coil conductors in the coil antennas.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an RFID (Radio FrequencyIdentification) system and, in particular, to a UHF band RFID systemused for short-distance communication between a reader/writer and anRFID tag.

2. Description of the Related Art

As a system for managing articles, an RFID system that establishes, onthe basis of a non-contact method, communication between a reader/writerand an RFID tag, and transmits information between the reader/writer andthe RFID tag is known. The RFID tag includes an RFIC chip used toprocess a wireless signal and an antenna used to transmit and receivethe wireless signal, and predetermined information is transmitted andreceived as a high-frequency signal between the antenna of the RFID tagand the antenna of the reader/writer through a magnetic field or a radiowave.

Since the RFID tag is to be attached to an article, a reduction in sizethereof is required.

As an RFID system that uses a small RFID tag, for example, a system isdisclosed in Japanese Unexamined Patent Application Publication No.2002-183676 where an RFID tag, in which a minute antenna coil is formedon a wireless IC chip, is utilized and by moving a resonance bodyincluding a capacitor and a coil, provided in a leading end portion of areader/writer, closer to this tag, information is read and written.

However, in the RFID system disclosed in Japanese Unexamined PatentApplication Publication No. 2002-183676, when the RFID tag is mounted toa mother substrate, such as printed wiring board, for example, the RFIDtag is influenced by a metallic substance, such as another mountedcomponent or a circuit pattern, for example, provided in the printedwiring board, and a communication distance is reduced or nocommunication is established. In addition, due to the influence of themetallic substance, the resonance frequency of the resonance bodydeviates and the transmission efficiency of a high-frequency signal isreduced, in some cases. In particular, when a metallic body is locatedadjacent to a portion in which the RFID tag is disposed, the deviationsof the resonance frequency and the reduction in transmission efficiencybecomes significant.

SUMMARY OF THE INVENTION

To overcome the problems described above, preferred embodiments of thepresent invention provide an RFID system that is capable of maintaininga communication distance and that is superior in terms of thetransmission efficiency of a high-frequency signal, even if it ismounted in a mother substrate.

In an RFID system according to a preferred embodiment of the presentinvention, preferably, a loop antenna including a loop conductor is usedas an antenna on a reader/writer side, a coil antenna including aplurality of laminated coil conductors is used as an antenna on an RFIDtag side, and a conductor width of the loop conductor in the loopantenna is greater than a conductor width of the coil conductor in thecoil antenna.

Since the loop antenna defined by the loop conductor is used as thereader/writer-side antenna, and the conductor width of the loopconductor in the loop antenna is greater than the conductor width of thecoil conductor in the coil antenna, it is possible to concentrate amagnetic flux on the center line of a winding axis in the loop antennawith a conductor loss in the loop antenna being reduced. In addition,since the loop of the magnetic flux becomes large, it is possible toradiate the magnetic flux farther. Furthermore, since the coil antennaformed by laminating the plural coil conductors is used as the RFIDtag-side antenna, and the conductor width of the coil conductors isrelatively small, it is possible to reduce a stray capacitance componentoccurring between the coil antenna and a metallic substance in a mothersubstrate, and it is possible to minimize the influence of the metallicsubstance. Accordingly, it is possible to improve the degree of couplingbetween the RFID tag and the reader/writer, and it is possible tomaintain a sufficient communication distance. Therefore, it is possibleto provide an RFID system that is superior in terms of favorabletransmission efficiency of a high-frequency signal.

According to various preferred embodiments of the present invention, itis possible to provide an RFID system that is capable of maintaining asufficient communication distance and that is superior in terms of thetransmission efficiency of a high-frequency signal.

The above and other elements, features, steps, characteristics, andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating an RFID system according to apreferred embodiment of the present invention.

FIGS. 2A and 2B illustrate the RFID system, wherein FIG. 2A is a planview showing a positional relationship between a reader/writer-sideantenna and an RFID tag-side antenna, and FIG. 2B is a side view.

FIG. 3 is an exploded view of a power feeding substrate in the RFIDsystem.

FIGS. 4A and 4B illustrate the reader/writer-side antenna in the RFIDsystem, wherein FIG. 4A is a perspective view from a back surface side,and FIG. 4B is a circuit diagram.

FIG. 5 is an explanatory diagram illustrating a usage pattern of theRFID system.

FIG. 6 is an explanatory diagram illustrating an operating principle inthe RFID system.

FIG. 7 is an explanatory diagram illustrating an operating principle inthe RFID system.

FIG. 8 is a plan view illustrating an example of a modification to thereader/writer-side antenna in the RFID system.

FIG. 9 is a cross-sectional view illustrating an RFID tag according to asecond example of a preferred embodiment of the present invention.

FIG. 10 is an explanatory diagram illustrating a magnetic fieldradiation state of the RFID tag illustrated in FIG. 9.

FIG. 11 is an exploded perspective view illustrating a laminatedstructure of the RFID tag illustrated in FIG. 9.

FIG. 12 is a cross-sectional view illustrating an RFID tag according toa third example of a preferred embodiment of the present invention.

FIG. 13 is an explanatory diagram illustrating a magnetic fieldradiation state of the RFID tag illustrated in FIG. 12.

FIG. 14 is an exploded perspective view illustrating a laminatedstructure of the RFID tag illustrated in FIG. 12.

FIG. 15 is a cross-sectional view illustrating an RFID tag according toa fourth example of a preferred embodiment of the present invention.

FIG. 16 is an explanatory diagram illustrating a magnetic fieldradiation state of the RFID tag illustrated in FIG. 15.

FIG. 17 is an exploded perspective view illustrating a laminatedstructure of the RFID tag illustrated in FIG. 15.

FIG. 18 is a cross-sectional view illustrating an RFID tag according toa fifth example of a preferred embodiment of the present invention.

FIG. 19 is an explanatory diagram illustrating a magnetic fieldradiation state of the RFID tag illustrated in FIG. 18.

FIG. 20 is a cross-sectional view illustrating an RFID tag according toa sixth example of a preferred embodiment of the present invention.

FIG. 21 is an explanatory diagram illustrating a magnetic fieldradiation state of the RFID tag illustrated in FIG. 20.

FIG. 22A is an explanatory diagram illustrating a magnetic fieldradiation state in an RFID system utilizing the RFID tag according tothe fifth example, and FIG. 22B is an explanatory diagram illustrating amagnetic field radiation state in an RFID system utilizing the RFID tagaccording to the sixth example of a preferred embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, RFID systems according to preferred embodiments of thepresent invention will be described with reference to accompanyingdrawings. In addition, in each drawing, a common reference character isassigned to the same component or the same portion, and redundantdescription thereof is omitted.

RFID System and First Example of RFID Tag

An RFID system according to a preferred embodiment is preferably asystem in which the transmission of information is performed between areader/writer and an RFID tag on the basis of a non-contact method, andmore specifically, is an RFID system in which the transmission andreception of a high-frequency signal of a UHF band or an SHF band isperformed between the antenna of a reader/writer and the antenna of anRFID tag, which are disposed so as to be adjacent to each other with adistance ranging from several mm to several cm therebetween.

First, with reference to FIGS. 1 to 4B, the configurations of areader/writer in the RFID system and an RFID tag as a first example of apreferred embodiment of the present invention will be described.

As illustrated in FIG. 1, the reader/writer preferably includes anantenna head 1 including a loop antenna 10 provided on the front surfaceof a supporting member 2. The loop antenna 10 is preferably defined by aloop conductor 11 of approximately one turn, whose power feeding endsare a power feeding portion 11 a and a power feeding portion 11 b, andthe power feeding portion 11 a and the power feeding portion 11 b areconnected to an information processing circuit in the main body of thereader/writer, not illustrated.

An RFID tag 20 preferably includes a coil antenna 30, formed bylaminating a plurality of coil conductors embedded in a power feedingsubstrate 40, and an RFIC element 50 connected to the coil antenna 30.The coil antenna 30 preferably includes a first coil antenna 31 and asecond coil antenna 32 adjacently disposed within the power feedingsubstrate 40 so that the winding axes of the coil antennas 31 and 32 areparallel or substantially parallel to each other, and the coil antennas31 and 32 are magnetically coupled to each other. In addition, whilebeing described hereinafter in detail, the coil antenna 30 preferablyfurther includes a third coil antenna 31. The coil antenna 30 isprovided within the power feeding substrate 40, and the power feedingsubstrate 40 is defined by a laminated body formed by laminating aplurality of dielectric layers. The power feeding substrate 40 ismounted on a mother substrate 60, such as a printed wiring board, forexample, and the RFIC element 50 is mounted on the front surface of thepower feeding substrate 40. The RFIC element 50 preferably includes alogic circuit, a memory circuit, and other suitable circuit elements,and is connected, as a bare IC chip or a packaged IC chip, to terminals36 and 37 on the power feeding substrate 40 through input-outputterminals on the back surface thereof.

As illustrated in FIGS. 2A and 2B, in planar view of the loop antenna 10on a reader/writer side and the coil antenna 30 on an RFID tag 20 side,an area occupied by the loop antenna 10 is approximately equal to orslightly greater than the total area of an area occupied by the coilantenna 30, namely, an area occupied by the first coil antenna 31 and anarea occupied by the second coil antenna 32. In addition, the areaoccupied by the loop antenna 10 is an area on an inner side with respectto a periphery of the loop antenna 10, and the area occupied by the coilantenna 30 is an area on an inner side with respect to the peripheriesof the coil antennas 31 and 32.

In addition, in the present preferred embodiment, the loop conductor 11of the loop antenna 10 is preferably wound on a plane surface, and thecoil conductors of the coil antennas 31 and 32 are preferably primarilywound in a lamination direction. Furthermore, the loop antenna 10 isconfigured such that the conductor width of the loop conductor 11 in theloop antenna 10 is preferably greater than the conductor widths of thecoil conductors in the coil antennas 31 and 32. Specifically, when theconductor width of the loop conductor 11 in the loop antenna is denotedas W1 and the conductor widths of the coil conductors of the coilantennas 31 and 32 are denoted as W2, the conductor width of the loopconductor 11 is set so as to satisfy a relationship of W1>W2.

As illustrated in FIG. 3, the power feeding substrate is preferablydefined by a laminated body formed by laminating a plurality of ceramicdielectric layers 41 a to 41 f, and includes therein the coil conductors31 a to 31 d and 32 a to 32 d and interlayer conductors 34 a to 34 e and35 a to 35 e of the coil antennas 31 and 32. Each of the ceramicdielectric layers 41 a to 41 f is preferably made of dielectricmaterial, such as LTCC (Low Temperature Co-fired Ceramic) material orother suitable material, whose relative permittivity ∈r is greater thanor equal to about 6, for example. The coil conductors 31 a to 31 d and32 a to 32 d and the interlayer conductors 34 a to 34 e and 35 a to 35 eare preferably made of low-melting-point metal material whose mainconstituent is copper or silver and whose resistivity is relativelysmall, for example. Particularly, the laminated body is preferablyobtained by simultaneously sintering a coil conductor or an interlayerconductor and a plurality of ceramic dielectric layers.

In the dielectric layer 41 a, the terminal 36 and the terminal 37 areprovided to be connected to two input-output terminals of the RFICelement 50, and the terminal 36 is connected to one end of the coilconductor 31 a, provided in the dielectric layer 41 b, through theinterlayer conductor 34 a provided in the dielectric layer 41 a. Thecoil conductor 31 a preferably has a small-diameter loop shape in thesurface of the dielectric layer 41 b, and the other end thereof isconnected to one end of the coil conductor 31 b provided in thedielectric layer 41 c, through the interlayer conductor 34 b provided inthe dielectric layer 41 b. The coil conductor 31 b preferably has asmall-diameter loop shape in the surface of the dielectric layer 41 c,and the other end thereof is connected to one end of the coil conductor31 c provided in the dielectric layer 41 d, through the interlayerconductor 34 c provided in the dielectric layer 41 c. The coil conductor31 c preferably has a small-diameter loop shape in the surface of thedielectric layer 41 d, and the other end thereof is connected to one endof the coil conductor 31 d provided in the dielectric layer 41 e,through the interlayer conductor 34 d provided in the dielectric layer41 d. The coil conductor 31 d preferably has a small-diameter loop shapein the surface of the dielectric layer 41 e, and the other end thereofis connected to one end of the coil conductor 33 a provided in thedielectric layer 41 f, through the interlayer conductor 34 e provided inthe dielectric layer 41 e.

Furthermore, the coil conductor 33 a preferably has a large-diameterloop shape in the surface of the dielectric layer 41 f, and the otherend thereof is connected to one end of the coil conductor 32 d providedin the dielectric layer 41 e, through the interlayer conductor 35 eprovided in the dielectric layer 41 e. The coil conductor 32 dpreferably has a small-diameter loop shape in the surface of thedielectric layer 41 e, and the other end thereof is connected to one endof the coil conductor 32 c provided in the dielectric layer 41 d,through the interlayer conductor 35 d provided in the dielectric layer41 d. The coil conductor 32 c preferably has a small-diameter loop shapein the surface of the dielectric layer 41 d, and the other end thereofis connected to one end of the coil conductor 32 b provided in thedielectric layer 41 c, through the interlayer conductor 35 c provided inthe dielectric layer 41 c. The coil conductor 32 b preferably has asmall-diameter loop shape in the surface of the dielectric layer 41 c,and the other end thereof is connected to one end of the coil conductor32 a provided in the dielectric layer 41 b, through the interlayerconductor 35 b provided in the dielectric layer 41 b. The coil conductor32 a preferably has a small-diameter loop shape in the surface of thedielectric layer 41 b, and the other end thereof is connected to theterminal 37 provided in the dielectric layer 41 a, through theinterlayer conductor 35 a provided in the dielectric layer 41 a.

Particularly, the first coil antenna 31 is configured using thesmall-diameter coil conductors 31 a to 31 d and the interlayerconductors 34 a to 34 e, and the second coil antenna 32 is configuredusing the small-diameter coil conductors 32 a to 32 d and the interlayerconductors 35 a to 35 e. Furthermore, in the present preferredembodiment, the third coil antenna 33 including the large-diameter coilconductor 33 a is also included. As illustrated in FIG. 3, the coilconductors 31 a to 31 d, 32 a to 32 d, and 33 a in the first coilantenna 31, the second coil antenna 32, and the third coil antenna 33are wound so that the directions of currents flowing in the individualcoil conductors are aligned in a same direction, i.e., the directions ofinduction magnetic fields generated by the currents flowing in theindividual coil conductors are aligned in a same direction.

Preferably, the first coil antenna 31, the second coil antenna 32, andthe third coil antenna 33 are adjacently disposed within the powerfeeding substrate 40 so that the winding axes of the individual coiledantennas are parallel or substantially parallel to one another, and thefirst coil antenna 31, the second coil antenna 32, and the third coilantenna 33 are magnetically coupled to one another. In addition, inplanar view, an area occupied by the loop antenna 10 on a reader/writerside is preferably approximately equal to the area of a portionsurrounded by the outside dimension of an antenna on an RFID tag 20side, i.e., the outside dimensions of the first coil antenna 31, thesecond coil antenna 32, and the third coil antenna 33.

As illustrated in FIG. 4A, the reader/writer-side antenna is preferablyconfigured as the loop antenna 10 provided in one main surface of theflat plate-shaped supporting member 2 including a rigid member, such asan epoxy resin, for example, and a coaxial cable 3 is connected to theother main surface of the supporting member 2. As illustrated in FIG.4B, between the loop antenna 10 and the coaxial cable 3, a matchingcircuit is preferably provided and includes a capacitance element C andan inductance element L, and the power feeding portion 11 a and thepower feeding portion 11 b in the loop antenna 10 are connected to aninternal conductor 4 of the coaxial cable 3 and an external conductor 5of the coaxial cable 3, respectively, through the matching circuit. Thecoaxial cable 3 is preferably configured as a 50Ω line, for example, anddue to the matching circuit, matching between the impedance of thecoaxial cable 3 and the impedance of the loop antenna 10 is achieved.

Next, with reference to FIG. 5 to FIG. 7, the usage pattern and theoperating principle of the RFID system of the present preferredembodiment will be described.

As illustrated in FIG. 5, the antenna head 1 in the reader/writerpreferably includes the loop antenna 10 provided in one main surface ofthe supporting member 2 and the matching circuit element including thecapacitance element C and the inductance element L provided in the othermain surface of the supporting member 2. This antenna head 1 isconnected to a gripper 6 through the coaxial cable 3, and is preferablyconfigured as a pen-shaped antenna capable of being used with thegripper 6 being gripped. The pen-shaped reader/writer-side antenna isfurther connected to a reader/writer main body, not illustrated, in a DCmanner or through a magnetic field or an electromagnetic field.

The RFID tag 20 preferably includes the rectangular or substantiallyrectangular flat plate-shaped power feeding substrate 40 and the RFICelement 50 mounted thereon, and the RFIC element 50 is sealed using asealing material 55, such as an epoxy resin, for example. This RFID tag20 is mounted to the mother substrate 60, such as a printed wiringboard, for example, through a joint material 56 including insulatingmaterial, such as resin, or conductive material, such as solder, forexample.

In the present preferred embodiment, as illustrated in FIG. 6, the loopantenna 10 is used as the reader/writer-side antenna, the coil antenna30 (31, 32) is used as the RFID tag 20-side antenna, and furthermore,the conductor width of the loop conductor 11 in the loop antenna 10 ispreferably greater than the conductor widths of the coil conductors 31 ato 31 d and 32 a to 32 d in the coil antennas 31 and 32. Therefore, in astate in which the antenna head 1 is adjacent to the RFID tag 20,magnetic fields H1, H2, H3, and H4 indicated by dashed lines in FIG. 6are generated, and through these magnetic fields, a high-frequencysignal is transmitted and received between the loop antenna 10 and thecoil antennas 31 and 32. Particularly, while maintaining a low conductorloss, the magnetic fields H1 and H2 generated due to the loop conductor11 of the loop antenna 10 cause an aperture portion magnetic field to beconcentrated and widely spread. In addition, as illustrated in FIG. 7,this magnetic flux interlinks with the coil conductors of the coilantennas 31 and 32.

In this manner, since, in the loop antenna 10 and the coil antennas 31and 32, it is possible to concentrate a magnetic field in each apertureportion, even if the mother substrate 60 is metal or metal is disposedin the vicinity of the mother substrate 60, it is possible for magneticfields to be intensively interlinked with the coil antennas 31 and 32.In addition, in the coil antennas 31 and 32, the conductor widths arenarrowed and laminated structures are utilized, and thus, it is possibleto focus the magnetic fields H3 and H4 primarily in a directionperpendicular or substantially perpendicular to the coil surfacesthereof. In addition to this, even if the mother substrate 60 is metal,capacitance occurring between the mother substrate 60 and the coilantennas 31 and 32 is relatively small, and has a small effect on aresonance frequency. Furthermore, since a high-frequency signal of theUHF band or a frequency band higher than the UHF band is utilized, evenif the RFID tag is mounted to the mother substrate 60, thehigh-frequency signal is not significantly influenced by another mountedcomponent mounted to the mother substrate 60 or metallic substances,such as various kinds of wiring patterns, for example, provided in themother substrate 60. In addition, as for the magnetic fields H1 and H2in the loop antenna 10, since the conductor width of the loop antenna 10is relatively wide, the magnetic fields H1 and H2 in the loop antenna 10widely spread primarily in a direction parallel or substantiallyperpendicular to the loop plane thereof, and even if the relativeposition of the loop antenna 10 in a planar direction with respect tothe RFID tag 20 somewhat deviates, the magnetic fields H1 and H2generated on the loop antenna 10 side easily interlink with the coilantennas 31 and 32, and an area in which reading and writing can beperformed is increased.

The RFID system of the present preferred embodiment is configured suchthat the reader/writer-side antenna and the RFID tag-side antenna aredisposed close to each other, and communication with the RFID tag 20that is a target of reading and writing for the reader/writer isestablished with only the RFID tag 20. In this case, for example, theoutside dimension of the coil antenna 10 is preferably less than orequal to about 1 cm long×about 1 cm wide, for example, and furthermore,it is possible to configure the coil antenna 10 in an extremely smallsize of less than or equal to about 0.5 cm long×about 0.5 cm wide, forexample. Specifically, for example, when an operation frequency band isa UHF band of about 860 MHz to about 960 MHz, the size of the powerfeeding substrate 40 is about 3.2 mm long×about 1.6 mm wide, the outsidedimension of the coil antenna 30 is about 2.5 mm long×about 1.2 mm wide,the outside dimension of the loop antenna 10 is about 3.0 mm long×about4.0 mm wide, the conductor width of the loop conductor 11 is about 0.5mm, and an output power value is about 1 W, it is possible to performreading and writing even if a distance between the reader/writer-sideantenna and the RFID tag-side antenna is about 6 mm. Further, byincreasing the output power value or increasing the size of the powerfeeding substrate 40, and more particularly, the size of the coilantenna 30, it is possible to further increase the communicationdistance.

While the present invention has been described with reference to aspecific preferred embodiment, the present invention is not limited tothe above-described configuration.

For example, as illustrated in FIG. 8, the reader/writer-side antenna isnot a loop antenna including one turn but may preferably be configuredusing the loop antenna 10 including a plurality of turns. In this case,a distance between the outermost diameter and the innermost diameter ofthe loop antenna 10 is the conductor width W1 of the loop conductor 11in the loop antenna 10. In addition, while it is preferable that theloop antenna 10 includes a loop-shaped single-layer conductor, the loopconductor may preferably include a plurality of layers if each of thelayers is thinner than the thickness of the coil antenna 30 in alamination direction.

As described above, it is preferable that the winding directions of thefirst coil antenna and the second coil antenna are the same. If coilconductors defining individual coil antennas are configured so that thewinding directions thereof are the same, currents in the individual coilantennas flow in the same direction, and induction magnetic fields dueto the currents are also generated in the same direction. Accordingly,the currents generated in the individual coil antennas do not canceleach other out, and the energy transmission efficiency of ahigh-frequency signal is improved. Therefore, a communication distancebetween the reader/writer-side antenna and the RFID tag-side antenna isincreased. In addition, the coil antennas have a laminated structure,and when the coil antennas are arranged in a position at which thewinding axes of two coil antennas overlap with each other in planarview, it is possible to enlarge the sum of opening areas in the coilantennas. As a result, since magnetic flux density increases, thecommunication distance further increases.

In addition, since the reader/writer-side antenna is a loop antenna andthe RFID tag-side antenna is a coil antenna, communication between thereader/writer and the RFID tag is performed primarily through a magneticfield. However, in this regard, if the conductor width of the loopconductor in the loop antenna is greater than the conductor width of thecoil conductor in the coil antenna, and a ratio between the outsidedimension of the loop antenna and the outside dimension of the coilantenna falls within a predetermined range, when the antenna head 1 andthe RFID tag are disposed at an extremely short distance from each otherso that the distance is less than or equal to about 2 mm, for example,capacitive coupling, in addition to the magnetic field coupling, isprovided. Accordingly, even if electric power is extremely small, it ispossible to perform wireless communication. Furthermore, preferably, theconductor width of the loop antenna is relatively wide, and theconductor width of the coil antenna on the RFID tag side is relativelynarrow. Therefore, even if the locations of the loop antenna and thecoil antenna deviate, the change of a capacitance value between the loopantenna and the coil antenna is very small, and accordingly, the changeof a characteristic is very small.

When it is intended to perform communication at such a short distance,it is preferable that the area occupied by the loop antenna of thereader/writer is about 0.2 to about 6 times as large as the areaoccupied by the coil antenna of the RFID tag. If the area occupied bythe loop antenna is less than an area about 0.2 times as large as thearea occupied by the coil antenna, it is difficult to fully transmit andreceive a high-frequency signal, in some cases. On the other hand, ifthe area occupied by the loop antenna is greater than an area about 6times as large as the area occupied by the coil antenna, it is difficultto concentrate the magnetic flux of the loop antenna and even if theloop antenna is arranged adjacent to the coil antenna, a region in whichit is difficult to perform reading and writing, namely, a null point,tends to occur. In addition, when the loop antenna is disposed at anextremely small distance, it is difficult for capacitive coupling tooccur.

In addition, it is preferable that the coil antenna is configured sothat the imaginary portion of the impedance of the RFIC element and theimaginary portion of the impedance of the coil antenna have a conjugaterelationship with each other at the operation frequency. Namely, it ispreferable that the coil antenna provided in the power feeding substratehas a function to match the impedance of the RFIC element in addition tohaving a function as an antenna. While the coil antenna has a resonancefrequency due to an inductance component of the coil itself and acapacitance component produced between lines, it is preferable that thisresonance frequency is located near the operation frequency. It isfurther preferable that the real portions of the impedances coincide orsubstantially coincide with each other. In particular, when an antennain which the first coil antenna and the second coil antenna aremagnetically coupled to each other is used as the coil antenna, theoperation frequency band may have a wider bandwidth.

In addition, the RFID tag may preferably be attached to the mothersubstrate using a bonding material, such as a double-stick tape, anadhesive material, or other suitable material, for example, and in thiscase, after being processed into a seal, a label, a tape, or other item,the RFID tag may also preferably be attached to the mother substrate. Atthis time, in the RFID tag, any one of the RFIC element side and thepower feeding substrate side thereof may be used as the surface to beattached to the mother substrate. In particular, if the RFIC element iscovered by a sealing material, it is possible to protect the RFICelement, and it is possible to attach the RFIC element to the mothersubstrate using the upper surface of the sealing material.

While a base material defining the power feeding substrate may alsopreferably be made of a typical resin material having relativepermittivity ∈r in a range of about 3 to about 4, if the power feedingsubstrate is configured using a material such as a ceramic dielectric,for example, whose relative permittivity ∈r is high, it is possible toachieve stable operation of the RFID system. Specifically, sinceline-line capacitance between the coil conductors is dependent upon thequality of material provided the coil conductors, the influence of therelative permittivity of the material used for the mother substrate isreduced, and fluctuations of stray capacitance is less likely to occur.In addition, the change of the inductance value of the coil conductor isalso relatively small. Therefore, the change of the resonance frequencyis small, and the communication distance is ensured, regardless of ausage environment.

While the RFID tag may preferably be mounted to various mothersubstrates, such as a printed wiring board, for example, the RFID tagmay also be mounted on a metallic plate. In this case, in the RFID tag,preferably, a surface on a side on which the RFIC element is provided isused as a mounting surface for a metallic plate, and a power feedingsubstrate side is used as a top surface side. Accordingly, communicationwith the reader/writer is more reliable. When the surface on the powerfeeding substrate side is used as the mounting surface for a metallicplate, the coil antenna is disposed as close to the upper side of thepower feeding substrate as possible. Accordingly, it is possible tosecure a path through which a magnetic flux passes, between the coilantenna and the metallic plate, and it is possible to stabilize anoperation on the metallic plate.

It is preferable that the antenna head in the reader/writer isconfigured so that the coaxial cable extends in a direction oblique tothe loop plane of the loop antenna and the coil plane of the coilantenna. By being configured in this manner, it is possible to reducemutual interference between both of the magnetic field generated in theloop antenna and the magnetic field generated in the coil antenna andthe coaxial cable.

In addition, it is only necessary for the coil antenna that is the RFIDtag-side antenna to be a lamination-type coil antenna including aplurality of laminated coil conductors in a direction perpendicular tothe loop plane of the loop antenna that is the reader/writer-sideantenna, and the coil antenna may also be a single coil antenna.

Second Example of RFID Tag

As illustrated in FIG. 9, an RFID tag 101A according to a second exampleof a preferred embodiment of the present invention preferably includesan RFIC element 110 arranged to process a transmission/reception signalof a predetermined frequency, a power feeding substrate 120, and a coilantenna 130 embedded in the power feeding substrate 120.

Preferably, the RFIC element 110 is configured in a chip form, includesa clock circuit, a logic circuit, a memory circuit, and other suitablecircuits, and stores necessary information, and a pair of input-outputterminal electrodes not illustrated are provided in the back surfacethereof. In addition, the RFIC element 110 is mounted on the powerfeeding substrate 120. The power feeding substrate 120 preferablyincludes a plurality of laminated layers whose main constituent isdielectric or magnetic material, for example.

As will be described hereinafter with reference to FIG. 11, the coilantenna 130 is preferably wound in a coil shape by laminating andconnecting conductor patterns 133 a to 133 c for a coil, formed onsheets 121 c to 121 e of dielectric or magnetic material, to each otherusing via hole conductors 134 a. Ends of the coil antenna 130 arepreferably electrically connected to the input-output terminalelectrodes of the RFIC element 110 through solder bumps 115, forexample.

As illustrated in FIG. 9, the RFID tag 101A is preferably attached to abase material 140, such as a printed wiring substrate or other suitablematerial, through an adhesive layer 141. The coil antenna 130 ispreferably embedded in the power feeding substrate 120 so that thecenter plane A of the antenna 130 in the lamination direction is locatedon a side opposite to the base material 140 with respect to the centerplane B of the power feeding substrate 120. Specifically, the coilantenna 130 is preferably located a distance C away from the frontsurface of the base material 140.

The RFID tag 101A is capable of communicating with a reader/writer in anRFID system, not illustrated, such that the RFID tag 101A and thereader/writer define an information processing system. In thisinformation processing system, by arranging the antenna of thereader/writer adjacent to the RFID tag 101A, a magnetic flux based on asignal of a predetermined frequency (for example, a UHF band or an HFband) radiated from the antenna penetrates the coil antenna 130, andthus a current flows in the antenna 130. This current is supplied to theRFIC element 110, thereby causing the RFIC element 110 to operate. Onthe other hand, a response signal from the RFIC element 110 is radiated,as a magnetic field, from the coil antenna 130, and read by thereader/writer.

A magnetic field H radiated from the coil antenna 130 is indicated bydotted lines in FIG. 10. Since the coil antenna 130 is embedded in thepower feeding substrate 120, and the center plane A of the antenna 130in the lamination direction is located on a side opposite to the basematerial 140, such as a printed wiring substrate or other suite B of thepower feeding substrate 120 in the lamination direction, the magneticfield H is primarily generated in a direction towards the antenna of thereader/writer and away from the base material 140. Therefore, aninfluence of a metallic substance, such as another mounted component, aconductor pattern, or other substance, for example, provided in the basematerial 140, is minimized, and the communication distance is notdecreased.

In addition, while communication between the reader/writer and the RFIDtag 101A is established primarily by a magnetic field, since theattenuation of the magnetic field with respect to a distance is greaterthan that of an electric field, communication is established in arelatively close state. Therefore, it is possible to establishcommunication with only the RFID tag to be a target to be read for thereader/writer, and there is little possibility that communication iserroneously established with a neighboring RFID tag that is not a targetto be read.

It is preferable that the imaginary portion of the impedance of the RFICelement 110 and the imaginary portion of the impedance of the coilantenna 130 have a conjugate relationship with each other at thefrequency of a signal used for communication. Particularly, it ispreferable that the resonance frequency of the coil antenna 130 islocated near the operation frequency. Further, it is preferable that thereal portions of the impedances coincide or substantially coincide witheach other.

In particular, when the coil antenna 130 is a lamination type coilantenna and has a relatively large aperture portion, it is possible toobtain a large inductance value with the size thereof being relativelysmall such that the overall size of the RFID tag 101A can be reduced. Bysetting the operation frequency to a short wavelength in the vicinity ofabout 950 MHz, for example, the size of the RFID tag 101A may be furtherreduced. When the frequency of a UHF band is used for communication, theRFID tag 101A may preferably have a size that is about 3.2 mm long,about 01.6 mm wide, and about 0.5 mm tall, for example.

Here, an example of the laminated structure of the power feedingsubstrate 120 (coil antenna 130) will be described with reference toFIG. 11. The power feeding substrate 120 is preferably obtained byforming and laminating electrodes, conductors, and via hole conductorsin a plurality of sheets 121 a to 121 e whose main constituent is adielectric material or a magnetic material, for example, andfurthermore, sheet groups 121 f and 121 g used to obtain the height ofthe center plane A are laminated therein.

Preferably, electrodes 131 a and 131 b to be connected to input-outputterminal electrodes of the RFIC element 110, not illustrated, andmounting electrodes 131 c and 131 d (to be connected to mountingterminal electrodes of the RFIC element 110, not illustrated) areprovided in the sheet 121 a of the first layer, connecting conductors132 a and 132 b are provided in the sheet 121 b of the second layer, andthe conductor patterns 133 a, 133 b, and 133 c for a coil are providedin the sheets 121 c to 121 e of the third layer to the fifth layer.

The conductor patterns 133 a, 133 b, and 133 c for a coil are connectedin a coil shape through via hole conductors 134 a, thereby forming theantenna 130. One end of the conductor pattern 133 a is connected to theelectrode 131 a through a via hole conductor 134 b, the connectingconductor 132 a, and a via hole conductor 134 c. In addition, one end ofthe conductor pattern 133 c is connected to the electrode 131 b througha via hole conductor 134 d, the connecting conductor 132 b, and a viahole conductor 134 e.

When a lamination type coil antenna 130 is provided, it is possible toachieve stable operation in addition to an enlarged aperture portion.Particularly, since capacitance between the conductor patterns 133 a,133 b, and 133 c for a coil is dependent on the quality of materialtherebetween (the quality of the material of the sheet), the influenceof the electric permittivity of the attachment target article of theRFID tag 101A is reduced (the fluctuation of stray capacitance is lesslikely to occur), and a change of the inductance value of the coil isminimized. Therefore, a change of the resonance frequency is minimized,and the communication distance ensured. In particular, by using amaterial having a high electric permittivity for the power feedingsubstrate 120, the impedance of the coil within the power feedingsubstrate 120 is accurately determined, and becomes insusceptible to ausage environment.

Third Example of RFID Tag

As illustrated in FIG. 12, an RFID tag 101B according to a third exampleof a preferred embodiment of the present invention is preferablyobtained by providing conductor patterns 133 a, 133 b, and 133 c for acoil in the upper portion of the power feeding substrate 120 to definethe coil antenna 130, mounting the RFIC element 110 on the back surfaceside of the power feeding substrate 120, and providing a sealing layer125 so as to cover the RFIC element 110. The upper surface of thesealing layer 125 is attached to the base material 140, such as aprinted wiring substrate or other suitable material, for example,through the adhesive layer 141.

In the RFID tag 101B, the coil antenna 130 is preferably embedded in thepower feeding substrate 120 so that the center plane A of the antenna130 in the lamination direction is located on a side opposite to thebase material 140 with respect to the center plane B of the powerfeeding substrate 120. Furthermore, the sealing layer 125 is disposedbetween the power feeding substrate 120 and the base material 140, andthus, the distance C between the coil antenna 130 and the front surfaceof the base material 140 greater than in the second example.

The operation of the RFID tag 101B is the same or substantially the sameas the second example and in particular, since the distance C isincreased, as illustrated by dotted lines in FIG. 13, the magnetic fieldH generated in the antenna 130 is farther away from the front surface ofthe base material 140 and is closer to the antenna of the reader/writer,not illustrated. Therefore, it is possible to more effectively eliminatethe influence of a metallic substance such as another mounted component,a conductor pattern, or other metallic substance, for example, providedin the base material 140. In addition, by covering the RFIC element 110using the sealing layer 125, the RFIC element 110 is protected from theexternal environment. Particularly, the RFIC element 110 is protectedfrom an external mechanical shock. In addition, it is possible toprevent a short circuit caused by moisture or other contaminants, forexample, from occurring.

The laminated structure of the power feeding substrate 120 (coil antenna30) in the present third example is illustrated in FIG. 14, the sheetgroups 121 f and 121 g illustrated in FIG. 11 are arranged between thesheet 121 b and the sheet 121 c, and the conductor pattern 133 c for acoil is provided in the back surface of the sheet 121 e. In addition,when the RFID tag 101B is attached to the base material 140, the sheet121 e is preferably an uppermost layer.

Fourth Example of RFID Tag

As illustrated in FIG. 15, an RFID tag 201A according to a fourthexample of a preferred embodiment of the present invention preferablyincludes the RFIC element 110 arranged to process atransmission/reception signal of a predetermined frequency, a powerfeeding substrate 220, and a coil antenna 230 embedded in the powerfeeding substrate 220.

As will be described hereinafter with reference to FIG. 17, the coilantenna 230 is preferably wound in a coil shape by laminating andconnecting conductor patterns 233 a to 233 c for a coil, provided onsheets 221 c to 221 e of dielectric or magnetic material, to each otherusing via hole conductors 234 a. Ends of the coil antenna 230 areelectrically connected to the input-output terminal electrodes of theRFIC element 110 through solder bumps 215, for example, respectively.

As illustrated in FIG. 15, the RFID tag 201A is preferably attached to abase material 240, such as a printed wiring substrate or other suitablematerial, for example, with an adhesive layer 241. As illustrated inFIG. 16, the coil antenna 230 is arranged so that the spread of amagnetic field H radiated from the antenna 230 varies depending on thetop surface side and the bottom surface side of the power feedingsubstrate 220. Specifically, the coil antenna 230 is preferably arrangedso that the opening sizes (meaning the internal diameters of individualpatterns in the present application) of the conductor patterns 233 a,233 b, and 233 c for a coil substantially increase in a direction fromthe bottom surface side of the power feeding substrate 220 to the topsurface side thereof.

The RFID tag 201A is capable of communicating with a reader/writer in anRFID system, not illustrated, and the RFID tag 201A and thereader/writing define an information processing system. In thisinformation processing system, by arranging the antenna of thereader/writer adjacent to the RFID tag 201A, a magnetic flux based on asignal of a predetermined frequency (for example, a UHF band or an HFband) radiated from the antenna penetrates the coil antenna 230, andthus, a current flows in the antenna 230. This current is supplied tothe RFIC element 110, thereby causing the RFIC element 110 to operate.On the other hand, a response signal from the RFIC element 110 isradiated, as a magnetic field, from the coil antenna 230, and read bythe reader/writer.

A magnetic field H radiated from the coil antenna 230 is indicated bydotted lines in FIG. 16. Since this coil antenna 230 is embedded in thepower feeding substrate 220, and the magnetic field H generated in thecoil antenna 230 spreads beyond the RFID tag 201A. Accordingly, thedegree of freedom of a positional relationship with the antenna of thereader/writer, not illustrated, is increased, and it is possible tostably establish communication over a wide range.

In addition, while communication between the reader/writer and the RFIDtag 201A is established primarily by a magnetic field, since theattenuation of the magnetic field with respect to a distance is greaterthan that of an electric field, communication is established in arelatively close state. Therefore, it is possible to establishcommunication with only the RFID tag to be a target to be read for thereader/writer, and there is little possibility that communication iserroneously established with a neighboring RFID tag that is not a targetto be read.

It is preferable that the imaginary portion of the impedance of the RFICelement 110 and the imaginary portion of the impedance of the coilantenna 230 have a conjugate relationship with each other at thefrequency of a signal used for communication. Particularly, it ispreferable that the resonance frequency of the coil antenna 230 islocated near the operation frequency. It is further preferable that thereal portions of the impedances coincide or substantially coincide witheach other.

In particular, when the coil antenna 230 is a lamination type coilantenna and has a relatively large aperture portion, it is possible toobtain a large inductance value with a relatively small coil antenna,and furthermore, the overall size of the RFID tag 201A can be reduced.By setting the operation frequency to a short wavelength in the vicinityof about 950 MHz, the size of the RFID tag 201A can be further reduced.When the frequency of a UHF band is used for communication, the RFID tag201A may preferably have a size of about 3.2 mm long, about 1.6 mm wide,and about 0.5 mm tall, for example.

Here, an example of the laminated structure of the power feedingsubstrate 220 (coil antenna 230) will be described with reference toFIG. 17. This power feeding substrate 220 is obtained by providing andlaminating electrodes, conductors, and via hole conductors in aplurality of sheets 221 a to 221 e whose main constituent is adielectric material or a magnetic material, for example.

Preferably, electrodes 231 a and 231 b, to be connected to input-outputterminal electrodes of the RFIC element 110, not illustrated, andmounting electrodes 231 c and 231 d (to be connected to mountingterminal electrodes of the RFIC element 110, not illustrated) areprovided in the sheet 221 a of the first layer, connecting conductors232 a and 232 b are provided in the sheet 221 b of the second layer, andconductor patterns 233 a, 233 b, and 233 c for a coil are provided inthe sheets 221 c to 221 e of the third layer to the fifth layer.

The conductor patterns 233 a, 233 b, and 233 c for a coil are connectedin a coil shape through via hole conductors 234 a, thereby defining theantenna 230. One end of the conductor pattern 233 a is connected to theelectrode 231 a through a via hole conductor 234 b, the connectingconductor 232 a, and a via hole conductor 234 c. In addition, one end ofthe conductor pattern 233 c is connected to the electrode 231 b througha via hole conductor 234 d, the connecting conductor 232 b, and a viahole conductor 234 e.

When a lamination type antenna coil is used for the coil antenna 230, itis possible to achieve stable operation in addition to an enlargedaperture portion. Particularly, since capacitance between the conductorpatterns 233 a, 233 b, and 233 c for a coil is dependent on the qualityof material therebetween (the quality of the material of the sheet), theinfluence of the electric permittivity of the attachment target articleof the RFID tag 201A is reduced (the fluctuation of stray capacitance isless likely to occur), and the change of the inductance value of thecoil is minimized. Therefore, a change of the resonance frequency isminimized, and the communication distance is ensured. In particular, byusing material having a high electric permittivity for the power feedingsubstrate 220, the impedance of the coil within the power feedingsubstrate 220 is effectively determined, and is not significantlyinfluenced by a usage environment.

In addition, the conductor patterns 233 a, 233 b, and 233 c for a coilin each coil antenna 230 may preferably be configured using a greaternumber of conductor patterns. In addition, it is only necessary for theopening size of each pattern to be configured so as to substantiallyincrease in a direction from the bottom surface side of the powerfeeding substrate 220 to the top surface side thereof. It is notnecessary for the term “substantially” to mean that the opening sizecontinuously increases in a step-by-step manner, and a conductor patternfor a coil, located midway, may also have the same opening size as thoseof patterns located above and below the conductor pattern oralternatively, the conductor pattern for a coil may also have an openingsize larger than that of a pattern located below the conductor pattern.In addition, in a preferred embodiment of the present inventiondescribed below, it is also not necessary for the opening size of eachpattern to continuously increases in a step-by-step manner in a verticaldirection.

Fifth Example of RFID Tag

As illustrated in FIG. 18, an RFID tag 201B according to a fifth exampleof a preferred embodiment of the present invention is preferablyobtained by mounting the RFIC element 110 on the back surface side ofthe power feeding substrate 220 in which the coil antenna 230 isembedded and providing a sealing layer 225 arranged to cover the RFICelement 110. The upper surface of the sealing layer 225 is attached tothe base material 240 such as a printed wiring substrate or othersuitable material, for example, by the adhesive layer 241.

The remaining configuration of the fifth example is the same orsubstantially the same as the fourth example. Accordingly, thefunctional effect of the fifth example is the same or substantially thesame as the fourth example. In particular, by covering the RFIC element110 using the sealing layer 225, the RFIC element 110 is protected froman external environment. Particularly, the RFIC element 110 is protectedfrom an external mechanical shock. In addition, it is possible toprevent a short circuit caused by moisture or other contaminants, forexample, from occurring. Furthermore, the sealing layer 225 ispreferably disposed between the power feeding substrate 220 and the basematerial 240. Therefore, the distance C between the coil antenna 230 andthe front surface of the base material 240 is greater than in the fourthexample, and the magnetic field H radiated from the coil antenna 230 isarranged so as to be farther away from the base material 240 (refer toFIG. 19). Therefore, it is possible to reduce the influence on themagnetic field H of a metallic substance, such as another mountedcomponent, a wiring pattern, or other metallic substance, for example,provided in the base material 240.

Sixth Example of RFID Tag

As illustrated in FIG. 20, an RFID tag 201C according to a sixth exampleof a preferred embodiment of the present invention is preferablyobtained by providing the conductor patterns 233 a, 233 b, and 233 c fora coil, which define the coil antenna 230, so that the opening sizesthereof substantially decrease in a direction from the bottom surfaceside of the power feeding substrate 220 to the top surface side thereof.In addition, in the same or substantially the same manner as the fourthexample, the RFIC element 110 is mounted to the bottom surface of thepower feeding substrate 220 and coated by the sealing layer 225.

The remaining configuration of the sixth example is the same orsubstantially the same as the fourth example. In the sixth example,since the magnetic field H is radiated from the coil antenna 230 asindicated by dotted lines in FIG. 21, and the opening sizes of theconductor patterns 233 a, 233 b, and 233 c are configured so as todecrease in a direction to the top surface of the power feedingsubstrate 220, the magnetic field H faces the inward side of the RFIDtag 201C, and a region whose magnetic flux density is relatively largeis provided in a central portion of the RFID tag 201C, thereby improvinga communication characteristic. In addition, the other functionaleffects of the sixth example are preferably the same or substantiallythe same as those of the fourth and fifth examples.

Magnetic Field Radiation State in Fifth Example and Sixth Example

In the RFID system according to preferred embodiments of the presentinvention, the loop antenna 10 is preferably used as areader/writer-side antenna, and as an RFID tag-side antenna, forexample, the coil antenna 230 is preferably used that is configured sothat most of a magnetic field is generated on the reader/writer side.Accordingly, in a state in which the antenna head 1 is arranged to beadjacent to the RFID tags 201B and 201C, as illustrated in FIGS. 22A and22B, the magnetic field H1 generated from the loop antenna 10 and themagnetic field H2 generated from the coil antenna 230 interlink witheach other, a high-frequency signal is mutually transmitted between theloop antenna 10 and the coil antenna 230.

The magnetic field H1 generated from the loop antenna is preferablyconcentrated in the aperture portion of the loop antenna 10, and widelyspread. On the other hand, in the coil antenna 230 in each of the RFIDtags 201B and 201C, since most of the magnetic field H2 is generated onthe reader/writer side, the magnetic field H2 is preferably concentratedin the aperture portion of the loop antenna 10. Accordingly, even if ametal material is included in or adjacent to the base material 240 oralternatively, the base material 240 is a metal material, communicationperformance is not deteriorated. Furthermore, by utilizing ahigh-frequency signal of the UHF band or a frequency band higher thanthe UHF band, even if the base material 240, on or in which each of theRFID tags 201B and 201C is mounted, is a metal material, thecommunication performance is not significantly influenced by anothermounted component mounted to the base material 240 or metal members,such as various kinds of wiring patterns, for example. In addition, evenif the RFID tags 101A, 101B, 201A, 201B, and 201C are used, since, fromthe coil antennas 130 and 230, most of the magnetic fields are generatedon the loop antenna 10 side of the reader/writer, the above-describedadvantageous effects are achieved.

In addition, when the conductor width of the loop antenna 10 isincreased, the magnetic field H1 generated from the loop antenna 10spreads widely primarily in a direction parallel or substantiallyparallel to the loop plane thereof, and even if the relative position ofthe loop antenna 10 with respect to the RFID tags 201B and 201C slightlydeviates, the magnetic fields H1 and H2 reliably interlink with eachother. Therefore, it is possible to ensure necessary communicationperformance. Since the magnetic field H2 generated in the coil antenna230 spreads outward when viewed from the RFID tags 201B and 201C, thedegree of freedom of a positional relationship with the loop antenna 10is significantly increased. In addition, in FIGS. 22A and 22B, themagnetic fields H1 and H2 do not graphically illustrate all of thegenerated magnetic fields.

In the RFID system according to preferred embodiments of the presentinvention, the antenna of the reader/writer and the RFID tag arepreferably used in a state of being adjacent to each other, and it ispossible to establish communication with only a target RFID tag. In thiscase, it is preferable to configure the coil antenna 230 with an outsidedimension thereof being less than or equal to an outside dimension ofabout 10 mm long and about 10 mm wide and more preferably with a smallsize of less than or equal to a size of about 5 mm long and 5 about mmwide. Specifically, when it is assumed that an operation frequency bandis a UHF band of about 860 MHz to about 960 MHz, the size of each of theRFID tags 201B and 201C is preferably about 3.2 mm long×about 1.6 mmwide, the outside dimension of the coil antenna 230 is preferably about3.0 mm long×about 4.0 mm wide, the conductor width of the loop antenna10 is preferably about 0.5 mm, and an output power value preferably isabout 1 W, for example, it is possible to establish communication evenif a distance between the loop antenna 10 and each of the RFID tags 201Band 201C is about 6 mm. By increasing the output power value orenlarging the size of the coil antenna 230, it is possible to furtherincrease the communication distance.

As described above, preferred embodiments of the present invention areuseful for an RFID system in which an RFID tag and a reader/writerestablish communication with each other with a distance therebetween ofseveral mm to several cm and in particular, preferred embodiments of thepresent invention are capable of maintaining a communication distanceand superior transmission efficiency of a high-frequency signal.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

What is claimed is:
 1. An RFID system comprising: a reader/writerincluding an antenna; and an RFID tag including an antenna attached toan article; wherein transmission and reception of a high-frequencysignal is performed between the antenna of the reader/writer and theantenna of the RFID tag; the antenna of the reader/writer is defined bya planar-shaped loop antenna including a loop conductor disposed in aloop plane; the antenna of the RFID tag is defined by athree-dimensional-shaped coil antenna including a plurality of coilconductors having coil surfaces; a conductor width of the loop conductorin the planar-shaped loop antenna is greater than a conductor width ofeach of the plurality of coil conductors in the three-dimensional-shapedcoil antenna; an outside dimension of the loop conductor in theplanar-shaped loop antenna is greater than an outside dimension of theplurality of coil conductors in the three-dimensional-shaped coilantenna, such that in the three-dimensional-shaped coil antenna, largermagnetic fields are generated in a direction perpendicular orsubstantially perpendicular to the coil surfaces than in a directionparallel or substantially parallel to the coil surfaces, and in theplanar-shaped loop antenna larger magnetic fields are generated in adirection parallel or substantially parallel to the loop plane than in adirection perpendicular or substantially perpendicular to the loopplane; and the plurality of coil conductors include portions that arestacked and wound so as to have a three-dimensional shape.
 2. The RFIDsystem according to claim 1, wherein the three-dimensional-shaped coilantenna is provided in a power feeding substrate including a pluralityof laminated dielectric layers.
 3. The RFID system according to claim 2,wherein the power feeding substrate includes a ceramic laminated body;and each of the plurality of laminated dielectric layers is a ceramicdielectric layer.
 4. The RFID system according to claim 1, wherein thethree-dimensional-shaped coil antenna includes a first coil antenna anda second coil antenna; and the first coil antenna and the second coilantenna are magnetically coupled to each other.
 5. The RFID systemaccording to claim 1, wherein the outside dimension of thethree-dimensional-shaped coil antenna is less than or equal to 1 cm×1 cmin planar view.
 6. The RFID system according to claim 5, wherein an areaoccupied by the planar-shaped loop antenna is about 0.2 to about 6 timesas large as an area occupied by the three-dimensional-shaped coilantenna, in planar view.
 7. The RFID system according to claim 1,wherein the high-frequency signal is a signal of a UHF band or an SHFband.
 8. The RFID system according to claim 1, wherein a largestdimension along outermost edges of the planar-shaped loop antenna isgreater than a largest dimension along outermost edges of thethree-dimensional-shaped coil antenna; the antenna of the reader/writerand the antenna of the RFID tag are configured to transmit and receiveUHF signals; the plurality of coil conductors of thethree-dimensional-shaped coil antenna are disposed on a plurality ofdielectric layers that are laminated to one another; and outsidedimensions of the coil antenna are less than or equal to 1 cm×1 cm. 9.The RFID system according to claim 1, wherein the loop conductor of theplanar-shaped loop antenna is defined by a single-layer loop conductor.10. The RFID system according to claim 1, wherein the loop conductor ofthe planar-shaped loop antenna consists of approximately one turn.